2009 Control System Feature Wishlist

[jhersh]

Originally Posted by Daniel_LaFleur

This actually surprises me, considering the number of USB DAQ devices ( like this, and this) entering the market. Attaching a simple driver circuit (either voltage or current) to one of the USB DAQ DOs allows for control of most pneumatic systems and some DC motors (single directional / single speed) while the limited AOs can control many of the more sophisticated motor systems.

Not to mention the sheer number of other USB devices that the cRio could control (Enviromental control and monitoring, control feedback through DAQs, Logic instrumentation, ocilliscope signals, etc) and all through a common port (latency, timing, and bandwidth would be an issue on the higher end devices).

This would seem to me to be an easy way to expand the cRio's capabilities while keeping the cost to the end user down.

JMHO

It is true that there are many USB data acquisition systems out there, but there is a reason they are not used for control. The bus latency and non-determinism are detrimental to control systems above around 250Hz. The USB only tries to send data to or from the device every 1ms (125us for high speed) or less, depending on bus utilization. Fundamentally, that is why they are called data acquisitions systems and the cRIO is called a programmable automation controller.

You'll notice that even for the cSeries IO modules, we have different chassis to use them in, such as the NI cDAQ-9172 chassis. This chassis is USB and can interact with the same modules that come in the kit. This is for reading lots of data very fast or writing lots of data very fast. It is not made for the read-compute-write single point data path that is needed for control systems.

I hope this clarifies the reason why USB is not used for control. If you have more questions, I'd be glad to attempt to answer them.

Cheers!
-Joe